Bundle aggregations outside of the serialization method. (#11612)

This makes the serialization of events synchronous (and it no longer access the database), but we must manually calculate and provide the bundled aggregations. Overall this should cause no change in behavior, but is prep work for other improvements.
2024-10-01 01:36:05 -04:00 · 2022-01-07 09:10:46 -05:00 · 2022-01-07 09:10:46 -05:00 · 6bf81a7a61
commit 6bf81a7a61
parent 6c68e874b1
17 changed files with 248 additions and 155 deletions
--- a/changelog.d/11612.misc
+++ b/changelog.d/11612.misc
@ -0,0 +1 @@
+Avoid database access in the JSON serialization process.
--- a/synapse/events/utils.py
+++ b/synapse/events/utils.py
@ -14,17 +14,7 @@
 # limitations under the License.
 import collections.abc
 import re
-from typing import (
-    TYPE_CHECKING,
-    Any,
-    Callable,
-    Dict,
-    Iterable,
-    List,
-    Mapping,
-    Optional,
-    Union,
-)
+from typing import Any, Callable, Dict, Iterable, List, Mapping, Optional, Union

 from frozendict import frozendict

@ -32,14 +22,10 @@ from synapse.api.constants import EventContentFields, EventTypes, RelationTypes
 from synapse.api.errors import Codes, SynapseError
 from synapse.api.room_versions import RoomVersion
 from synapse.types import JsonDict
-from synapse.util.async_helpers import yieldable_gather_results
 from synapse.util.frozenutils import unfreeze

 from . import EventBase

-if TYPE_CHECKING:
-    from synapse.server import HomeServer
-
 # Split strings on "." but not "\." This uses a negative lookbehind assertion for '\'
 # (?<!stuff) matches if the current position in the string is not preceded
 # by a match for 'stuff'.
@ -385,17 +371,12 @@ class EventClientSerializer:
    clients.
    """

-    def __init__(self, hs: "HomeServer"):
-        self.store = hs.get_datastore()
-        self._msc1849_enabled = hs.config.experimental.msc1849_enabled
-        self._msc3440_enabled = hs.config.experimental.msc3440_enabled
-
-    async def serialize_event(
+    def serialize_event(
        self,
        event: Union[JsonDict, EventBase],
        time_now: int,
        *,
-        bundle_aggregations: bool = False,
+        bundle_aggregations: Optional[Dict[str, JsonDict]] = None,
        **kwargs: Any,
    ) -> JsonDict:
        """Serializes a single event.
@ -418,66 +399,41 @@ class EventClientSerializer:
        serialized_event = serialize_event(event, time_now, **kwargs)

        # Check if there are any bundled aggregations to include with the event.
-        #
-        # Do not bundle aggregations if any of the following at true:
-        #
-        # * Support is disabled via the configuration or the caller.
-        # * The event is a state event.
-        # * The event has been redacted.
-        if (
-            self._msc1849_enabled
-            and bundle_aggregations
-            and not event.is_state()
-            and not event.internal_metadata.is_redacted()
-        ):
-            await self._injected_bundled_aggregations(event, time_now, serialized_event)
+        if bundle_aggregations:
+            event_aggregations = bundle_aggregations.get(event.event_id)
+            if event_aggregations:
+                self._injected_bundled_aggregations(
+                    event,
+                    time_now,
+                    bundle_aggregations[event.event_id],
+                    serialized_event,
+                )

        return serialized_event

-    async def _injected_bundled_aggregations(
-        self, event: EventBase, time_now: int, serialized_event: JsonDict
+    def _injected_bundled_aggregations(
+        self,
+        event: EventBase,
+        time_now: int,
+        aggregations: JsonDict,
+        serialized_event: JsonDict,
    ) -> None:
        """Potentially injects bundled aggregations into the unsigned portion of the serialized event.

        Args:
            event: The event being serialized.
            time_now: The current time in milliseconds
+            aggregations: The bundled aggregation to serialize.
            serialized_event: The serialized event which may be modified.

        """
-        # Do not bundle aggregations for an event which represents an edit or an
-        # annotation. It does not make sense for them to have related events.
-        relates_to = event.content.get("m.relates_to")
-        if isinstance(relates_to, (dict, frozendict)):
-            relation_type = relates_to.get("rel_type")
-            if relation_type in (RelationTypes.ANNOTATION, RelationTypes.REPLACE):
-                return
+        # Make a copy in-case the object is cached.
+        aggregations = aggregations.copy()

-        event_id = event.event_id
-        room_id = event.room_id
-
-        # The bundled aggregations to include.
-        aggregations = {}
-
-        annotations = await self.store.get_aggregation_groups_for_event(
-            event_id, room_id
-        )
-        if annotations.chunk:
-            aggregations[RelationTypes.ANNOTATION] = annotations.to_dict()
-
-        references = await self.store.get_relations_for_event(
-            event_id, room_id, RelationTypes.REFERENCE, direction="f"
-        )
-        if references.chunk:
-            aggregations[RelationTypes.REFERENCE] = references.to_dict()
-
-        edit = None
-        if event.type == EventTypes.Message:
-            edit = await self.store.get_applicable_edit(event_id, room_id)
-
-        if edit:
+        if RelationTypes.REPLACE in aggregations:
            # If there is an edit replace the content, preserving existing
            # relations.
+            edit = aggregations[RelationTypes.REPLACE]

            # Ensure we take copies of the edit content, otherwise we risk modifying
            # the original event.
@ -502,27 +458,19 @@ class EventClientSerializer:
            }

        # If this event is the start of a thread, include a summary of the replies.
-        if self._msc3440_enabled:
-            (
-                thread_count,
-                latest_thread_event,
-            ) = await self.store.get_thread_summary(event_id, room_id)
-            if latest_thread_event:
-                aggregations[RelationTypes.THREAD] = {
-                    # Don't bundle aggregations as this could recurse forever.
-                    "latest_event": await self.serialize_event(
-                        latest_thread_event, time_now, bundle_aggregations=False
-                    ),
-                    "count": thread_count,
-                }
+        if RelationTypes.THREAD in aggregations:
+            # Serialize the latest thread event.
+            latest_thread_event = aggregations[RelationTypes.THREAD]["latest_event"]

-        # If any bundled aggregations were found, include them.
-        if aggregations:
-            serialized_event["unsigned"].setdefault("m.relations", {}).update(
-                aggregations
+            # Don't bundle aggregations as this could recurse forever.
+            aggregations[RelationTypes.THREAD]["latest_event"] = self.serialize_event(
+                latest_thread_event, time_now, bundle_aggregations=None
            )

-    async def serialize_events(
+        # Include the bundled aggregations in the event.
+        serialized_event["unsigned"].setdefault("m.relations", {}).update(aggregations)
+
+    def serialize_events(
        self, events: Iterable[Union[JsonDict, EventBase]], time_now: int, **kwargs: Any
    ) -> List[JsonDict]:
        """Serializes multiple events.
@ -535,9 +483,9 @@ class EventClientSerializer:
        Returns:
            The list of serialized events
        """
-        return await yieldable_gather_results(
-            self.serialize_event, events, time_now=time_now, **kwargs
-        )
+        return [
+            self.serialize_event(event, time_now=time_now, **kwargs) for event in events
+        ]


 def copy_power_levels_contents(
--- a/synapse/handlers/events.py
+++ b/synapse/handlers/events.py
@ -119,7 +119,7 @@ class EventStreamHandler:

            events.extend(to_add)

-            chunks = await self._event_serializer.serialize_events(
+            chunks = self._event_serializer.serialize_events(
                events,
                time_now,
                as_client_event=as_client_event,
--- a/synapse/handlers/initial_sync.py
+++ b/synapse/handlers/initial_sync.py
@ -170,7 +170,7 @@ class InitialSyncHandler:
                d["inviter"] = event.sender

                invite_event = await self.store.get_event(event.event_id)
-                d["invite"] = await self._event_serializer.serialize_event(
+                d["invite"] = self._event_serializer.serialize_event(
                    invite_event,
                    time_now,
                    as_client_event=as_client_event,
@ -222,7 +222,7 @@ class InitialSyncHandler:

                d["messages"] = {
                    "chunk": (
-                        await self._event_serializer.serialize_events(
+                        self._event_serializer.serialize_events(
                            messages,
                            time_now=time_now,
                            as_client_event=as_client_event,
@ -232,7 +232,7 @@ class InitialSyncHandler:
                    "end": await end_token.to_string(self.store),
                }

-                d["state"] = await self._event_serializer.serialize_events(
+                d["state"] = self._event_serializer.serialize_events(
                    current_state.values(),
                    time_now=time_now,
                    as_client_event=as_client_event,
@ -376,16 +376,14 @@ class InitialSyncHandler:
            "messages": {
                "chunk": (
                    # Don't bundle aggregations as this is a deprecated API.
-                    await self._event_serializer.serialize_events(messages, time_now)
+                    self._event_serializer.serialize_events(messages, time_now)
                ),
                "start": await start_token.to_string(self.store),
                "end": await end_token.to_string(self.store),
            },
            "state": (
                # Don't bundle aggregations as this is a deprecated API.
-                await self._event_serializer.serialize_events(
-                    room_state.values(), time_now
-                )
+                self._event_serializer.serialize_events(room_state.values(), time_now)
            ),
            "presence": [],
            "receipts": [],
@ -404,7 +402,7 @@ class InitialSyncHandler:
        # TODO: These concurrently
        time_now = self.clock.time_msec()
        # Don't bundle aggregations as this is a deprecated API.
-        state = await self._event_serializer.serialize_events(
+        state = self._event_serializer.serialize_events(
            current_state.values(), time_now
        )

@ -480,7 +478,7 @@ class InitialSyncHandler:
            "messages": {
                "chunk": (
                    # Don't bundle aggregations as this is a deprecated API.
-                    await self._event_serializer.serialize_events(messages, time_now)
+                    self._event_serializer.serialize_events(messages, time_now)
                ),
                "start": await start_token.to_string(self.store),
                "end": await end_token.to_string(self.store),
--- a/synapse/handlers/message.py
+++ b/synapse/handlers/message.py
@ -246,7 +246,7 @@ class MessageHandler:
                room_state = room_state_events[membership_event_id]

        now = self.clock.time_msec()
-        events = await self._event_serializer.serialize_events(room_state.values(), now)
+        events = self._event_serializer.serialize_events(room_state.values(), now)
        return events

    async def get_joined_members(self, requester: Requester, room_id: str) -> dict:
--- a/synapse/handlers/pagination.py
+++ b/synapse/handlers/pagination.py
@ -537,14 +537,16 @@ class PaginationHandler:
                state_dict = await self.store.get_events(list(state_ids.values()))
                state = state_dict.values()

+        aggregations = await self.store.get_bundled_aggregations(events)
+
        time_now = self.clock.time_msec()

        chunk = {
            "chunk": (
-                await self._event_serializer.serialize_events(
+                self._event_serializer.serialize_events(
                    events,
                    time_now,
-                    bundle_aggregations=True,
+                    bundle_aggregations=aggregations,
                    as_client_event=as_client_event,
                )
            ),
@ -553,7 +555,7 @@ class PaginationHandler:
        }

        if state:
-            chunk["state"] = await self._event_serializer.serialize_events(
+            chunk["state"] = self._event_serializer.serialize_events(
                state, time_now, as_client_event=as_client_event
            )

--- a/synapse/handlers/room.py
+++ b/synapse/handlers/room.py
@ -1181,6 +1181,16 @@ class RoomContextHandler:
        # `filtered` rather than the event we retrieved from the datastore.
        results["event"] = filtered[0]

+        # Fetch the aggregations.
+        aggregations = await self.store.get_bundled_aggregations([results["event"]])
+        aggregations.update(
+            await self.store.get_bundled_aggregations(results["events_before"])
+        )
+        aggregations.update(
+            await self.store.get_bundled_aggregations(results["events_after"])
+        )
+        results["aggregations"] = aggregations
+
        if results["events_after"]:
            last_event_id = results["events_after"][-1].event_id
        else:
--- a/synapse/handlers/search.py
+++ b/synapse/handlers/search.py
@ -420,10 +420,10 @@ class SearchHandler:
        time_now = self.clock.time_msec()

        for context in contexts.values():
-            context["events_before"] = await self._event_serializer.serialize_events(
+            context["events_before"] = self._event_serializer.serialize_events(
                context["events_before"], time_now
            )
-            context["events_after"] = await self._event_serializer.serialize_events(
+            context["events_after"] = self._event_serializer.serialize_events(
                context["events_after"], time_now
            )

@ -441,9 +441,7 @@ class SearchHandler:
            results.append(
                {
                    "rank": rank_map[e.event_id],
-                    "result": (
-                        await self._event_serializer.serialize_event(e, time_now)
-                    ),
+                    "result": self._event_serializer.serialize_event(e, time_now),
                    "context": contexts.get(e.event_id, {}),
                }
            )
@ -457,7 +455,7 @@ class SearchHandler:
        if state_results:
            s = {}
            for room_id, state_events in state_results.items():
-                s[room_id] = await self._event_serializer.serialize_events(
+                s[room_id] = self._event_serializer.serialize_events(
                    state_events, time_now
                )

--- a/synapse/rest/admin/rooms.py
+++ b/synapse/rest/admin/rooms.py
@ -424,7 +424,7 @@ class RoomStateRestServlet(RestServlet):
        event_ids = await self.store.get_current_state_ids(room_id)
        events = await self.store.get_events(event_ids.values())
        now = self.clock.time_msec()
-        room_state = await self._event_serializer.serialize_events(events.values(), now)
+        room_state = self._event_serializer.serialize_events(events.values(), now)
        ret = {"state": room_state}

        return HTTPStatus.OK, ret
@ -744,22 +744,22 @@ class RoomEventContextServlet(RestServlet):
            )

        time_now = self.clock.time_msec()
-        results["events_before"] = await self._event_serializer.serialize_events(
+        results["events_before"] = self._event_serializer.serialize_events(
            results["events_before"],
            time_now,
-            bundle_aggregations=True,
+            bundle_aggregations=results["aggregations"],
        )
-        results["event"] = await self._event_serializer.serialize_event(
+        results["event"] = self._event_serializer.serialize_event(
            results["event"],
            time_now,
-            bundle_aggregations=True,
+            bundle_aggregations=results["aggregations"],
        )
-        results["events_after"] = await self._event_serializer.serialize_events(
+        results["events_after"] = self._event_serializer.serialize_events(
            results["events_after"],
            time_now,
-            bundle_aggregations=True,
+            bundle_aggregations=results["aggregations"],
        )
-        results["state"] = await self._event_serializer.serialize_events(
+        results["state"] = self._event_serializer.serialize_events(
            results["state"], time_now
        )

--- a/synapse/rest/client/events.py
+++ b/synapse/rest/client/events.py
@ -91,7 +91,7 @@ class EventRestServlet(RestServlet):

        time_now = self.clock.time_msec()
        if event:
-            result = await self._event_serializer.serialize_event(event, time_now)
+            result = self._event_serializer.serialize_event(event, time_now)
            return 200, result
        else:
            return 404, "Event not found."
--- a/synapse/rest/client/notifications.py
+++ b/synapse/rest/client/notifications.py
@ -72,7 +72,7 @@ class NotificationsServlet(RestServlet):
                "actions": pa.actions,
                "ts": pa.received_ts,
                "event": (
-                    await self._event_serializer.serialize_event(
+                    self._event_serializer.serialize_event(
                        notif_events[pa.event_id],
                        self.clock.time_msec(),
                        event_format=format_event_for_client_v2_without_room_id,
--- a/synapse/rest/client/relations.py
+++ b/synapse/rest/client/relations.py
@ -113,13 +113,14 @@ class RelationPaginationServlet(RestServlet):
        now = self.clock.time_msec()
        # Do not bundle aggregations when retrieving the original event because
        # we want the content before relations are applied to it.
-        original_event = await self._event_serializer.serialize_event(
-            event, now, bundle_aggregations=False
+        original_event = self._event_serializer.serialize_event(
+            event, now, bundle_aggregations=None
        )
        # The relations returned for the requested event do include their
        # bundled aggregations.
-        serialized_events = await self._event_serializer.serialize_events(
-            events, now, bundle_aggregations=True
+        aggregations = await self.store.get_bundled_aggregations(events)
+        serialized_events = self._event_serializer.serialize_events(
+            events, now, bundle_aggregations=aggregations
        )

        return_value = pagination_chunk.to_dict()
@ -308,7 +309,7 @@ class RelationAggregationGroupPaginationServlet(RestServlet):
        )

        now = self.clock.time_msec()
-        serialized_events = await self._event_serializer.serialize_events(events, now)
+        serialized_events = self._event_serializer.serialize_events(events, now)

        return_value = result.to_dict()
        return_value["chunk"] = serialized_events
--- a/synapse/rest/client/room.py
+++ b/synapse/rest/client/room.py
@ -642,6 +642,7 @@ class RoomEventServlet(RestServlet):
    def __init__(self, hs: "HomeServer"):
        super().__init__()
        self.clock = hs.get_clock()
+        self._store = hs.get_datastore()
        self.event_handler = hs.get_event_handler()
        self._event_serializer = hs.get_event_client_serializer()
        self.auth = hs.get_auth()
@ -660,10 +661,13 @@ class RoomEventServlet(RestServlet):
            # https://matrix.org/docs/spec/client_server/r0.5.0#get-matrix-client-r0-rooms-roomid-event-eventid
            raise SynapseError(404, "Event not found.", errcode=Codes.NOT_FOUND)

-        time_now = self.clock.time_msec()
        if event:
-            event_dict = await self._event_serializer.serialize_event(
-                event, time_now, bundle_aggregations=True
+            # Ensure there are bundled aggregations available.
+            aggregations = await self._store.get_bundled_aggregations([event])
+
+            time_now = self.clock.time_msec()
+            event_dict = self._event_serializer.serialize_event(
+                event, time_now, bundle_aggregations=aggregations
            )
            return 200, event_dict

@ -708,16 +712,20 @@ class RoomEventContextServlet(RestServlet):
            raise SynapseError(404, "Event not found.", errcode=Codes.NOT_FOUND)

        time_now = self.clock.time_msec()
-        results["events_before"] = await self._event_serializer.serialize_events(
-            results["events_before"], time_now, bundle_aggregations=True
+        results["events_before"] = self._event_serializer.serialize_events(
+            results["events_before"],
+            time_now,
+            bundle_aggregations=results["aggregations"],
        )
-        results["event"] = await self._event_serializer.serialize_event(
-            results["event"], time_now, bundle_aggregations=True
+        results["event"] = self._event_serializer.serialize_event(
+            results["event"], time_now, bundle_aggregations=results["aggregations"]
        )
-        results["events_after"] = await self._event_serializer.serialize_events(
-            results["events_after"], time_now, bundle_aggregations=True
+        results["events_after"] = self._event_serializer.serialize_events(
+            results["events_after"],
+            time_now,
+            bundle_aggregations=results["aggregations"],
        )
-        results["state"] = await self._event_serializer.serialize_events(
+        results["state"] = self._event_serializer.serialize_events(
            results["state"], time_now
        )

--- a/synapse/rest/client/sync.py
+++ b/synapse/rest/client/sync.py
@ -17,7 +17,6 @@ from collections import defaultdict
 from typing import (
    TYPE_CHECKING,
    Any,
-    Awaitable,
    Callable,
    Dict,
    Iterable,
@ -395,7 +394,7 @@ class SyncRestServlet(RestServlet):
        """
        invited = {}
        for room in rooms:
-            invite = await self._event_serializer.serialize_event(
+            invite = self._event_serializer.serialize_event(
                room.invite,
                time_now,
                token_id=token_id,
@ -432,7 +431,7 @@ class SyncRestServlet(RestServlet):
        """
        knocked = {}
        for room in rooms:
-            knock = await self._event_serializer.serialize_event(
+            knock = self._event_serializer.serialize_event(
                room.knock,
                time_now,
                token_id=token_id,
@ -525,21 +524,14 @@ class SyncRestServlet(RestServlet):
            The room, encoded in our response format
        """

-        def serialize(events: Iterable[EventBase]) -> Awaitable[List[JsonDict]]:
+        def serialize(
+            events: Iterable[EventBase],
+            aggregations: Optional[Dict[str, Dict[str, Any]]] = None,
+        ) -> List[JsonDict]:
            return self._event_serializer.serialize_events(
                events,
                time_now=time_now,
-                # Don't bother to bundle aggregations if the timeline is unlimited,
-                # as clients will have all the necessary information.
-                # bundle_aggregations=room.timeline.limited,
-                #
-                # richvdh 2021-12-15: disable this temporarily as it has too high an
-                # overhead for initialsyncs. We need to figure out a way that the
-                # bundling can be done *before* the events are stored in the
-                # SyncResponseCache so that this part can be synchronous.
-                #
-                # Ensure to re-enable the test at tests/rest/client/test_relations.py::RelationsTestCase.test_bundled_aggregations.
-                bundle_aggregations=False,
+                bundle_aggregations=aggregations,
                token_id=token_id,
                event_format=event_formatter,
                only_event_fields=only_fields,
@ -561,8 +553,21 @@ class SyncRestServlet(RestServlet):
                    event.room_id,
                )

-        serialized_state = await serialize(state_events)
-        serialized_timeline = await serialize(timeline_events)
+        serialized_state = serialize(state_events)
+        # Don't bother to bundle aggregations if the timeline is unlimited,
+        # as clients will have all the necessary information.
+        # bundle_aggregations=room.timeline.limited,
+        #
+        # richvdh 2021-12-15: disable this temporarily as it has too high an
+        # overhead for initialsyncs. We need to figure out a way that the
+        # bundling can be done *before* the events are stored in the
+        # SyncResponseCache so that this part can be synchronous.
+        #
+        # Ensure to re-enable the test at tests/rest/client/test_relations.py::RelationsTestCase.test_bundled_aggregations.
+        # if room.timeline.limited:
+        #    aggregations = await self.store.get_bundled_aggregations(timeline_events)
+        aggregations = None
+        serialized_timeline = serialize(timeline_events, aggregations)

        account_data = room.account_data

--- a/synapse/server.py
+++ b/synapse/server.py
@ -759,7 +759,7 @@ class HomeServer(metaclass=abc.ABCMeta):

    @cache_in_self
    def get_event_client_serializer(self) -> EventClientSerializer:
-        return EventClientSerializer(self)
+        return EventClientSerializer()

    @cache_in_self
    def get_password_policy_handler(self) -> PasswordPolicyHandler:
--- a/synapse/storage/databases/main/relations.py
+++ b/synapse/storage/databases/main/relations.py
@ -13,14 +13,30 @@
 # limitations under the License.

 import logging
-from typing import List, Optional, Tuple, Union, cast
+from typing import (
+    TYPE_CHECKING,
+    Any,
+    Dict,
+    Iterable,
+    List,
+    Optional,
+    Tuple,
+    Union,
+    cast,
+)

 import attr
+from frozendict import frozendict

-from synapse.api.constants import RelationTypes
+from synapse.api.constants import EventTypes, RelationTypes
 from synapse.events import EventBase
 from synapse.storage._base import SQLBaseStore
-from synapse.storage.database import LoggingTransaction, make_in_list_sql_clause
+from synapse.storage.database import (
+    DatabasePool,
+    LoggingDatabaseConnection,
+    LoggingTransaction,
+    make_in_list_sql_clause,
+)
 from synapse.storage.databases.main.stream import generate_pagination_where_clause
 from synapse.storage.relations import (
    AggregationPaginationToken,
@ -29,10 +45,24 @@ from synapse.storage.relations import (
 )
 from synapse.util.caches.descriptors import cached

+if TYPE_CHECKING:
+    from synapse.server import HomeServer
+
 logger = logging.getLogger(__name__)


 class RelationsWorkerStore(SQLBaseStore):
+    def __init__(
+        self,
+        database: DatabasePool,
+        db_conn: LoggingDatabaseConnection,
+        hs: "HomeServer",
+    ):
+        super().__init__(database, db_conn, hs)
+
+        self._msc1849_enabled = hs.config.experimental.msc1849_enabled
+        self._msc3440_enabled = hs.config.experimental.msc3440_enabled
+
    @cached(tree=True)
    async def get_relations_for_event(
        self,
@ -515,6 +545,98 @@ class RelationsWorkerStore(SQLBaseStore):
            "get_if_user_has_annotated_event", _get_if_user_has_annotated_event
        )

+    async def _get_bundled_aggregation_for_event(
+        self, event: EventBase
+    ) -> Optional[Dict[str, Any]]:
+        """Generate bundled aggregations for an event.
+
+        Note that this does not use a cache, but depends on cached methods.
+
+        Args:
+            event: The event to calculate bundled aggregations for.
+
+        Returns:
+            The bundled aggregations for an event, if bundled aggregations are
+            enabled and the event can have bundled aggregations.
+        """
+        # State events and redacted events do not get bundled aggregations.
+        if event.is_state() or event.internal_metadata.is_redacted():
+            return None
+
+        # Do not bundle aggregations for an event which represents an edit or an
+        # annotation. It does not make sense for them to have related events.
+        relates_to = event.content.get("m.relates_to")
+        if isinstance(relates_to, (dict, frozendict)):
+            relation_type = relates_to.get("rel_type")
+            if relation_type in (RelationTypes.ANNOTATION, RelationTypes.REPLACE):
+                return None
+
+        event_id = event.event_id
+        room_id = event.room_id
+
+        # The bundled aggregations to include, a mapping of relation type to a
+        # type-specific value. Some types include the direct return type here
+        # while others need more processing during serialization.
+        aggregations: Dict[str, Any] = {}
+
+        annotations = await self.get_aggregation_groups_for_event(event_id, room_id)
+        if annotations.chunk:
+            aggregations[RelationTypes.ANNOTATION] = annotations.to_dict()
+
+        references = await self.get_relations_for_event(
+            event_id, room_id, RelationTypes.REFERENCE, direction="f"
+        )
+        if references.chunk:
+            aggregations[RelationTypes.REFERENCE] = references.to_dict()
+
+        edit = None
+        if event.type == EventTypes.Message:
+            edit = await self.get_applicable_edit(event_id, room_id)
+
+        if edit:
+            aggregations[RelationTypes.REPLACE] = edit
+
+        # If this event is the start of a thread, include a summary of the replies.
+        if self._msc3440_enabled:
+            (
+                thread_count,
+                latest_thread_event,
+            ) = await self.get_thread_summary(event_id, room_id)
+            if latest_thread_event:
+                aggregations[RelationTypes.THREAD] = {
+                    # Don't bundle aggregations as this could recurse forever.
+                    "latest_event": latest_thread_event,
+                    "count": thread_count,
+                }
+
+        # Store the bundled aggregations in the event metadata for later use.
+        return aggregations
+
+    async def get_bundled_aggregations(
+        self, events: Iterable[EventBase]
+    ) -> Dict[str, Dict[str, Any]]:
+        """Generate bundled aggregations for events.
+
+        Args:
+            events: The iterable of events to calculate bundled aggregations for.
+
+        Returns:
+            A map of event ID to the bundled aggregation for the event. Not all
+            events may have bundled aggregations in the results.
+        """
+        # If bundled aggregations are disabled, nothing to do.
+        if not self._msc1849_enabled:
+            return {}
+
+        # TODO Parallelize.
+        results = {}
+        for event in events:
+            event_result = await self._get_bundled_aggregation_for_event(event)
+            if event_result is not None:
+                results[event.event_id] = event_result
+
+        return results
+

 class RelationsStore(RelationsWorkerStore):
    pass
--- a/tests/rest/client/test_retention.py
+++ b/tests/rest/client/test_retention.py
@ -228,7 +228,7 @@ class RetentionTestCase(unittest.HomeserverTestCase):
        self.assertIsNotNone(event)

        time_now = self.clock.time_msec()
-        serialized = self.get_success(self.serializer.serialize_event(event, time_now))
+        serialized = self.serializer.serialize_event(event, time_now)

        return serialized
				`@ -0,0 +1 @@`
				`Avoid database access in the JSON serialization process.`